Schematically, the conventional direct current electric machines are formed by a stator and a rotor, both of which are made of iron; generally, the stator supports the inductor windings, whilst the rotor supports the induced windings.
The stator costitutes almost always the outer part of the machine and generally it is formed by a ring (or core), towards the inside of which the poles, each provided with excitation windings made of copper, project.
The rotor, usually of massive structure owing to the excitation by direct current, is a rolled-iron cylinder keyed on a shaft resting on supports. Formed in the peripheral zone of the rotor, near the air gap, are stator slots in which the induced winding is housed. This latter is connected, by means of conductors, to a cylinder-shaped commutator, with segments insulated with respect to each other, also inserted into the shaft. Resting on the outer surface of the cylinder are the brushes having, extending therefrom, the conductors leading to the terminals of the machine. The operation of this conventional machine being well-known to those skilled in the art, will not be described herein.
As can be seen, this machine requires various components, the most delicate of which structurally are the rotor provided with a winding, the commutator with segments made of copper and insulated from each other, and the brushes formed by blocks of conductive material. All this implies a careful working of the various components, a quite high cost thereof and a constant maintenance of the machine to always keep each component thereof in a perfectly efficient condition so as to maintain the efficiency of the machine itself.